Earthquake magnitude is a measure of the energy released during the earthquake and defines the size of the seismic event.Accepted measure of magnitude is the Richter scale and relationship between energy released, E, and Richter magnitude, M, is
log10 E=11.4 + 1.5M
Where E is in ergs.
Where E is in ergs.
Richter scale magnitude is calculated from the maximum amplitude ‘A’ of the seismometer trace as shown in the figure below
Figure 1 
 A0 is the seismometer reading produced by an Earthquake of standard size (i.e., a calibration earthquake). Generally A0 is 0.001 mm.
 This equation assumes that a distance of 100 km separates the seismometer and the epicentre. For other distances the nomograph and the following procedure can be used to calculate the magnitude.
 Due to lack of reliable information on the nature of the Earth between the observation point and the earthquake epicentre, an error of 10 to 40 km in locating the epicentre is not unrealistic.
Richter Scale Magnitude Calculation Steps
Note: One seismometer can determine the approximate distance to the epicentre, it takes three seismometers to determine and verify the location of the epicentre.

Figure 2 Richter magnitude correction nomograph
 Step2: Determine the maximum amplitude of the oscillation.(figure1)
 Step3: Connect the arrival time difference on the left scale and the amplitude on the right scale with a straight line.(figure 2)
 Step4: Read the Richter magnitude on the centre scale.(figure 2)
 Step5: Read the distance separating the seismometer and the epicentre from the left scale.(figure 2)